The ${prefix}/lib/libboost_python-mt.dylib (& related) library has a dependency on the system python library, not on the python2.5 installed by macports (obviously only when installing boost with port install boost +python25). This can be seen using eg:

$> otool -L lib/libboost_python-mt.dylib
lib/libboost_python-mt.dylib:
/opt/local/lib/libboost_python-mt.dylib (compatibility version 0.0.0, current version 0.0.0)
/System/Library/Frameworks/Python.framework/Versions/2.5/Python (compatibility version 2.5.0, current version 2.5.1)
/usr/lib/libstdc++.6.dylib (compatibility version 7.0.0, current version 7.4.0)
/usr/lib/libgcc_s.1.dylib (compatibility version 1.0.0, current version 1.0.0)
/usr/lib/libSystem.B.dylib (compatibility version 1.0.0, current version 111.0.0)

The problem only really shows up when compiling a source to a .so module with boost-python: the macports python2.5 complains when importing the module:
"Fatal Python error: Interpreter not initialized (version mismatch?)", while the system python does not show this error.

(Note that printing sys.path in macports python2.5 does not show any standard system path, only the macports one, as it should.)

The problem does not exist for the python 2.6 (probably since there are no system libraries for this version).